The purpose of this investigation is to study the physiological adaptations which occur in skeletal muscle as a consequence of exposure to hypoxia. Specifically, the effects of hypoxia on skeletal muscle fiber composition and on muscle capillary distribution will be determined by histological and histochemical technics with the aid of stereology. The functional relationship between changes in capillary density and muscle blood flow will be studied by radioisotopes: in dogs the fractional systemic blood flow distribution will be studied by means of radioactive nuclides incorporated in carbonized microspheres and in small rodents muscle blood flow will be determined by the washout technic. To evaluate the changes in oxygenation which may occur at the cellular level as a consequence of changes in capillary density and fiber composition, intracellular Po2 measurements will be obtained by microelectrodes.